The challenges of global climate change and energy security demands have made the development of renewable energy alternatives vital for our future. Globally, wind turbines account for only about two percent of electricity generated—but it is clear that this fraction needs to grow in order to break the chains of petroleum dependency.
In order to produce electricity from a wind turbine, rotation of the wind turbines is transferred to an alternator, typically directly or through a transmission gear system. The alternator outputs an electromagnetic force (hereinafter “EMF”) that is proportional to the revolutions per minute (hereinafter “RPM”) of the alternator and to the strength of a magnetic field generated by relative movement of magnets to electromagnetic assemblies including coils wound around magnetic cores.
The need to capture more wind power, i.e., improve the conversion of the rotation of the wind turbine into electrical energy, is leading to an increase in the span of the turbine blades of the wind turbine. This, in turn, leads to lower RPM of the wind turbine. Consequently, a conventional wind turbine includes a transmission gear section or gearbox that increases the RPM of the alternator. Some of the main disadvantages of this type of transmission gear section or gearbox include: energy losses, lower overall efficiency, and higher weight and maintenance.
Reliability issues with wind turbine gearboxes are known to exist and stem, for example, from the extreme engineering challenges that gearbox technology faces in wind applications. In order to reduce failure and downtime, the industry is shifting toward direct drive generator technology that eliminates the need for a gearbox.
Thus, a wind turbine without a transmission gear section is often referred to as a direct drive wind turbine. A direct drive wind turbine alternator has a larger diameter, and there are two primary types of such alternators, a radial flux type and an axial flux type.
The present invention relates to axial flux alternators that usually include magnets and electromagnetic assemblies that move relative to one another and have an air gap therebetween. Constructing the alternator to provide a desired dimension for the air gap between the magnets and the coils of an axial flux alternator, also referred to herein as adjusting the air gap, and maintaining the air gap during operation of the wind turbine, is a technical challenge, particularly for a large diameter axial flux alternator.
Another issue of concern for wind turbines is that large wind turbine span leads to a lower RPM's. In order to achieve high frequency with low RPM's, a large diameter direct drive generator is required. Consequently, direct drive generators are very heavy. For example, the direct-drive generator used in Enercon's 6 MW turbine weighs about 450 tons. At such weights, problems rapidly develop with transportation and installation, particularly with the availability of lifting ships capable of deploying such weights in an offshore environment. This, in turn, can lead to dramatic increases in offshore wind project installation costs as well as the overall cost effectiveness of such a project.
Furthermore, in order to achieve sufficient frequency with low RPM's, a relatively large number of magnets is required. As a result, the magnetic attraction forces between the large number of installed magnets and a correspondingly large number of coil-wound magnetic cores creates high resistance to rotation. For projects located far from the grid connection point, or of several hundred megawatts in capacity, AC transmission becomes costly or impossible, due to cable-generated reactive power using up much of the transmission capacity. In such cases, high voltage DC (HVDC) transmission is becoming a viable option. While voltage-source high voltage DC transmission is a relatively new commercial technology, as most future offshore wind farms will be large and/or located far from shore, HVDC is expected to be widely used going forward.
Still another issue of concern for direct drive generators is that they are expected to reduce down time and maintenance cost. However, as direct drive permanent magnet generators typically comprise neodymium-iron-boron elements, there are major concerns over how rare earth minerals could limit the development of direct drive wind turbine technologies. The vast majority of rare earth metals is being produced in China, of which about 43.5% of the world's supply comes from a single mine in the town of Bayan Obo in Baotou. For the sixth consecutive year, the Chinese Ministry of Industry and Information Technology has trimmed output quotas of rare earth metals, making the resource scarce and expensive. Consequently, minimizing the rare earth magnets used per each megawatt generated will not only significantly reduce transportation and installation costs due to lower weight but will also lead the direct drive wind turbine sector toward sustainable development and growth.